// Paper.java (Immutable)
public class Paper {
    private final String title;

    public Paper(String title) {
        if (title == null || title.length() < 3) {
            throw new IllegalArgumentException("论文标题长度至少3个字符");
        }
        this.title = title;
    }

    public String getTitle() {
        return title;
    }

    @Override
    public String toString() {
        return getClass().getSimpleName() + "(" + title + ")";
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Paper paper = (Paper) o;
        return title.equals(paper.title);
    }

    @Override
    public int hashCode() {
        return title.hashCode();
    }
}


